1. Field of the Invention
The present invention is in the field of mass spectrometers and more particularly is concerned with an improved structure and method for producing a mass filter for use in a mass spectrometer.
2. Description of the Prior Art
A type of mass spectrometer comprises an ion source, a mass filter and an ion collector disposed along a common axis is an evacuatable enclosure. A gas to be analyzed is introduced to the evacuatable chamber in the vicinity of the ion source where it is ionized by the ion source. These ions are attracted towards the ion collector by an impressed electrical potential, but the mass filter permits only ions having a predetermined mass-to-charge ratio to reach the collector. The current produced by the collected ions is measured for various predetermined mass-to-charge ratios to define the mass spectrum.
The mass filter typically comprises at least two electrodes disposed along the common axis, to which both radio frequency and D.C. voltages are applied. Various electrode configurations have been used, but the most widely used appears to be the quadrupole design, which has four electrodes opposing each other, in pairs, symmetrically about the common axis. Another mass filter design that has been used is called a monopole configuration, which actually has two electrodes. Still other configurations employ auxiliary electrodes. The construction and method herein disclosed are not limited to any particular electrode configuration.
In operation, the voltages impressed on the electrodes are typically on the order of several thousand volts. Typically, the electrode spacing is only a few millimeters or less. A mass spectrometer can operate on lower voltage and less power if its electrodes are close together, but this is possible only if its parts can be made and positioned with extreme accuracy. The combination of high voltage and close spacing gives rise to several problems which have not been completely solved in the art to date. A first problem, common to many high voltage systems, is that unless the electrode surfaces are extremely smooth, corona discharge from the electrodes can seriously affect the performance of the instrument. Charge build-up on the unshielded portions of the insulators can distort the electric field. Finally, the shape and relative positioning of the electrodes must be controlled with great accuracy to produce the desired quality of electrical field between the electrodes.
In each of these problem areas an art has grown up with the goal of solving the particular problem and further increasing the performance of the instrument.
It was found that the problem of charge build-up on the insulators could be bypassed by shaping the electrodes in such a manner as to shield the insulators from the active cavity or to minimize the insulator area exposed to the active cavity.
With regard to the problem of fabricating the electrodes, it was early recognized (see U.S. Pat. No. 2,939,952 to W. Paul, et al.) that although a hyperbolic shape was preferred for the electrodes, a practical approximation could be achieved through the use of electrodes having a circular cross-section instead. This greatly reduced the expense of machining the hyperbolic shape although that shape remains preferable. In addition to having the proper shape, the electrodes must have a very smooth surface finish, and the use of polished rods for electrodes is well known.
More difficult than the problem of electrode shape has been the problem of accurately positioning the electrodes with respect to each other. The accuracy desired here is on the order of 1/10,000th of an inch. Unless the instrument is carefully designed, the slightest mechanical shock can disturb the interelectrode spacings, thereby seriously degrading the performance of the mass spectrometer.
The earliest technique for mounting the electrodes is that shown in the patent to Paul, et al., referred to above. Paul shows a disc-like insulator of mica or ceramic disposed perpendicular to the common axis and having holes in it through which the ends of the rod-like electrodes pass. This arrangement is limited, of course, by the precision with which the several holes in the several discs can be machined and registered.
A different approach to the problem of mounting the electrodes is shown in the U.S. Pat. No. 3,553,451 in which the electrodes are drawn by screws into contact with the inside surface of a single large hollow ceramic insulator. An alternative approach to the positioning problem is to provide auxiliary electrodes as shown in U.S. Pat. No. 3,725,700 to Turner, for the purpose of favorably modifying the characteristics of the electric field.
Because of the necessity for precisely positioning the many parts of the mass filters constructed by the prior art methods, it can readily be appreciated that the assembly and adjusting of such units was a complex and expensive undertaking, generally requiring iterative adjustments. The present invention obviates these problems entirely.
In U.S. Pat. No. 3,328,146 to Hanlein there is taught a method for producing an electrode system for mass spectrometers. In that method, a tube of glass, plastic or tetrafluoroethylene is softened by heating so that it conforms to the shape of a stainless steel male mandrel. Upon cooling, the mandrel is removed and portions of the inner surface of the tube are metallized by vaporizing or sputtering gold onto them. The metallized portions are the electrodes.
It is apparent that the Hanlein method produces gold electrodes which are backed by a layer of glass or plastic, substances not noted for their dimensional stability during temperature changes and whose thermal expansion coefficients do not match that of gold. Further, the metal will tend to magnify any surface irregularities and contribute its own irregularities and variations in deposition thickness, as was the case with the Ball invention cited above.
A new departure from many of the above problems was disclosed in U.S. Pat. No. 3,757,115 to Ball. Ball shows a mass filter comprising a cylindrical ceramic body having an axial passage, the sides of the passage being fairly accurate hyperbolic surfaces which are plated to form electrodes. However, in the Ball structure and method, the deposition process will both tend to magnify any surface irregularities and contribute also its own irregularities and variations in deposition thickness. The working surface of the plated electrodes is not the relatively accurate surface of the axial passage but rather depends for its accuracy on the production of a smooth and uniform plating on the ceramic surface.
The Ball method requires that a separate cylindrical ceramic body be prepared for each mass filter fabricated whereas in the present invention, many mass filters can be produced from a single precisely dimensioned mandrel.
The present invention retains the advantages of the Ball structure, such as one-piece construction and true hyperbolic surfaces while overcoming Ball's dependency on the accuracy of plating process, and the relative difficulty in obtaining dimensional accuracy and repeatability in fired ceramics.